Rett syndrome (RTT) is an X-linked dominant neurodevelopmental disorder causing a profound loss of motor and speech development at about 6 to 18 months of age in females. The recent discovery of mutations in the MECP2 gene in patients with RTT has explained the underlying genetic basis for the syndrome, but opened many additional questions about the disease pathogenesis. MECP2 encodes for the protein MeCP2 that selectively binds to methylated CpG residues and is hypothesized to globally silence gene transcription. Parentally imprinted genes are also regulated by methylation and are essential in mammalian development. Surprisingly, MECP2-is ubiquitously expressed at the transcriptional level, although RIT appears to only exhibit abnormalities in the post-natal central nervous system (CNS). Therefore, understanding the expression and regulation of MeCP2 in the brain will be essential to understanding RIT as well as devising potential early intervention therapies. The PI has previously demonstrated that MeCP2 shows cellular and regional heterogeneity in protein expression levels in the cerebral cortex, cerebellum, and hippocampus by quantitative immunofluorescence with a novel automated analysis approach by laser scanning cytometry (LSC). The goal of the current study is to test the hypothesis that elevated MeCP2 is acquired in a subpopulation of cells during postnatal brain development and required for nuclear organization during the development of mature neurons. The aims are 1) to quantitate the developmental expression pattern of MeCP2 in human and mouse tissue microarrays and neuronal cultures, 2) to determine the effect of MECP2/Mecp2 mutations in Rett brain and Mecp2-null mice on the nuclear and cellular phenotype of cerebral cortical neurons, 3) to determine if transcriptional or translational controls determine the cellular and developmental heterogeneity in the level of MeCP2 expression, and 4) to test a role for elevated MeCP2 expression on the nuclear organization of parentally imprinted chromosomes. These results from these studies are expected to be of significance in understanding the role of MeCP2 in normal human brain development and in understanding the pathogenesis of Rett syndrome.